A temperature-dependent phosphorus doping on TiCT MXene for enhanced supercapacitance.

A novel type of phosphorus doped TiCT MXene nanosheets (P-TiCT) is synthesized via a facile and controllable strategy of annealing MXene nanosheets with the presence of sodium hypophosphite. A combination of theoretical density functional theory calculation and experimental X-ray photoelectron spectroscopy discloses that the doped P atoms are prone to fill into Ti vacancies first due to their lowest formation free energy (ΔG = -0.028 eV·Å) and next to bond with surface terminals on MXene layers (ΔG = 0.013 eV·Å), forming P-C and P-O species, respectively. More importantly, the as-obtained P-TiCT is, for the first time, investigated as the electrode material for supercapacitors, demonstrating a significantly boosted electrochemical performance by P doping. As a result, P-TiCT electrode delivers a high specific capacitance of 320 F·g at a current density of 0.5 A·g (much higher than 131 F·g for undoped MXene), an ultrahigh rate retention of 83.8% capacitance at 30 A·g, and a high cycling stability over continuous 5000 cycles.